使用compareTo表示年，月和日。Java

Question

I wanna sort the dates out on the screen but the thing is, when I use compareTo method, I can only use it with one of the Date attributes: Year, month or day.. not all of them. This is a custom class, not java.util.Date class.

I am required to sort it out on the screen using compareTo method.

This is what I have done.

public int compareTo(Date date) {
    if(getYear()-date.getYear()>0 && getMonth()-date.getMonth()>0){

        return 1;
    }
    if(getYear()-date.getYear()<0 && getMonth()-date.getMonth()<0 ){

        return -1;
    }
    return 0;
}

Answer 1

You should only compare the months if the years are equal. Similarly, you should only compare the days if the months are equal.

public int compareTo(Date date) {
    if(getYear() == date.getYear()) {
        if (getMonth() == date.getMonth()) {
            return getDay() - date.getDay ();
        } else {
            return getMonth() - date.getMonth ();
        }
    } else {
        return getYear() - date.getYear();
    }
}

Note that if for a given property the values of the two Dates are not equal, you can return the difference of those values instead of checking if it's positive or negative and then returning 1 or -1. compareTo is not required to return 1, 0 or -1. It can return any int value.

Answer 2

Java 8 alleviates some of the boilerplate for sorting by particular fields with Comparator.comparing .

dateList.sort(Comparator.comparing(Date::getYear)
                  .thenComparing(Date::getMonth)
                  .thenComparing(Date::getDay));